Choosing correct prop

Discussion in 'Props' started by Shannon Billman, Jun 25, 2019.

  1. Shannon Billman
    Joined: Jun 2019
    Posts: 1
    Likes: 0, Points: 1
    Location: Curtice,Ohio

    Shannon Billman New Member

    I recently restored a 1938 57 foot steel tug up here on lake Erie.had to work on this boat for 6 months 2 days a week 4 and 1/2 hours from my home. I build Marine engines for a living so I took the natural 671 Detroit and rebuilt the engine. It has a 3 to 1 reduction gear.Specs 57ft 13beam .Boat draws 5ft6in. 20ton.Has a natural 671 Detroit with a 3to1 reduction gear. This boat had an ancient steel prop on it I would say 34 x34. I sent it to Henry Smith who refurbishes props for my business just to clean it up enough for me to make the trip home. did not want to spend a ton of money on a prop until the boat show me it was worthy of it. Now that I have it at home the boat needs a new propeller. wouldn't even consider the one that's on it to use it for any ideas or dimensions. The boat Will easily red line with that prop on it like the prop was just slipping away and wasn't going anywhere. I drove the boat home and ran the boat 24 hours a day and it took me three and a half days to get the boat home 540 mile trip. The only thing I could imagine being correct on this prop is the diameter. I would say it's only 3 in from hitting the bottom of the boat when it spins so definitely nothing over 34 in diameter.the pitch I am clueless what it should be. When it comes to propellers I am in outer space I build engines and know absolutely nothing about the prop that would be perfect for this boat.Thanks
     

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    Last edited: Jun 25, 2019
  2. jehardiman
    Joined: Aug 2004
    Posts: 3,067
    Likes: 571, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Were there any markings on the prop? Do you know the Blade Area Ratio? How many blades? Is it keyed on the shaft? Was it installed properly with a pilgrim nut? Was the prop overspinning with no cavitation, or were you just digging a hole in the water? What was does your speed to rpm curve look like? There are a lot of questions to answerer before we can propose a solution. Your prop may have the wrong area, be pitched wrong, or as it is steel, so messed up it is cavatating, or so worn/ improperly installed it is slipping on the shaft. Was it built as a fish tug from the beginning (Fish tug - Wikipedia https://en.wikipedia.org/wiki/Fish_tug) or was it converted from a conventional tug boat? Things like that make a difference.
     
  3. JSL
    Joined: Nov 2012
    Posts: 811
    Likes: 62, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    jehardiman has some good comments & questions.
    More information or a complete list of specs would really help: ie:...what is the HP rating of the engine ( ? BHP at ? RPM), is the bottom clean, etc etc.
    Two suspicious comments:
    (a) diameter/pitch ratio appears relatively high for a slower boat
    (b) Tip clearance of 3" is way too small - should be at least 15% dia. (5"), preferably 20%
    Sounds like someone increased the pitch to make up for insufficient diameter: Does not work
     
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